SEASONAL-RELATED, TIDAL-CYCLE-RELATED AND MICROHABITAT-RELATED VARIATION IN MEMBRANE ORDER OF PHOSPHOLIPID-VESICLES FROM GILLS OF THE INTERTIDAL MUSSEL MYTILUS-CALIFORNIANUS

Alterations in the order, or fluidity, of cellular membranes in respon se to variations in environmental temperature are well known, The muss el Mytilus californianus, a common inhabitant of mid-intertidal region s along the Pacific coast of North America, can experience large (20 d egrees C or more) and cyclic (every 6 h) changes in body temperature ( T-b) during tidal cycles. In the present study, we explore membrane or der during seasonal and tidal temperature cycles and find that vesicle s prepared from gill phospholipids exhibit significant seasonal differ ences in order that are consistent with homeoviscous adaptation and su ggest winter-acclimatization to mean T-b values and summer-acclimatiza tion to upper extreme T-b values or to large cyclic thermal fluctuatio ns, despite repeated resubmergence in sea water at 10 degrees C during both seasons. Phospholipid vesicles prepared from the gills of mussel s acclimated for 6 weeks to constant high or low temperatures in the l aboratory fail to exhibit temperature-compensatory differences in orde r, In addition, during the summer, mussels inhabiting high intertidal sites, but not those from low sites, possess the ability to alter memb rane order rapidly (within hours), This alteration of order appears to represent a mechanism designed to offset the thermal variations encou ntered during the tidal cycle, Thus, M. californianus have the ability to adjust membrane order on seasonal as well as hourly time scales an d do so on the basis of their height in the intertidal region.